4G ALE Harris Spectrum Sensing

A p p l ic a t io n o f Sp e c t r u m Se n s i n g   t o w a r d a W id e b a n d H F A L E So l u t i o n  

William William F urm urman, Eric Eric Koski, Koski, J ohn ohn Nieto Nieto Harris Corporation / RF Communications

Pr e s e n t a t i o n o v e r v ie w  

• Wideband HF waveforms • Wideband ALE concept •  The need for spectrum sensing • Wideband ALE spectrum sensing requirements • Spectrum sensing prototype • Spectrum sensing: field test observations • Contribution to use of wideband HF waveforms • Caveats • Conclusions and future work

W id e b a n d H F w a v e fo r m s   • New appendix in MIL-STD-188-110C that defines – Suite of waveforms from 3 kHz to 24 kHz at intervals of 3 kHz – Bit rates of 75bps to 120000bps – Many modes • 8 bandwidths • 13 waveform modulations • 4 Interleaver settings • Adjustable preamble length

• Standard created collaboratively between Harris and Rockwell Collins • Benefits – Higher capacity/throughput when conditions allow – More robustness when there is a performance advantage to going with a wider bandwidth and simpler constellation

W id e b a n d A L E c o n c e p t  

• Establish links suitable for use of the wideband waveforms • Use 3 kHz signaling of existing ALE systems – For ease and cost-effectiveness of implementation and adoption

• Must coordinate selection of wider bandwidth to be used for data transmission – Waveforms lack any auto-bandwidth capability

• Support adaptive selection of other waveform parameters (modulation, coding)

T h e n e e d fo r s p e c t r u m s e n s in g  

• We can apparently get frequency allocations of up to 24 kHz • In practice, these are often at least partially occupied – Allocations are often provided for use on a ‘noninterfering basis’

•  The multiple bandwidths of the WBHF waveform family can allow us to effectively use part of an allocated channel • A spectrum sensing capability would allow us to identify the usable portion of an occupied wideband channel

P r e c e d e n t s f o r ‘s p e c t r u m   sensing’  • “Listen Before Transmit” has been a feature of the 2G and 3G ALE standards • In both cases, it’s intended for collision avoidance as part of a Media Access Control (MAC) function • LBT makes a binary determination (busy/not-busy) for an entire 3 kHz channel AWGN 3 kHz Minimum Required Waveform SNR (dB)

Detection Probability

0

50%

6

90%

-9

50%

-6

95%

0

30%

6

70%

6

50%

9

75%

MIL-STD-188-110 or

0

30%

FED-STD-1052 PSK modem

6

70%

STANAG 4285 or

0

30%

STANAG 4529 PSK modem

6

70%

2G-ALE Robust LSU (BW0) HDL (BW2) single sideband (SSB) Voice

W id e b a n d A L E s p e c t r u m   s e n s in g r e q u ir e m e n t s   • Capable of examining a band of at least 24 kHz width • Detect presence of interference in the band that might preclude effective use of the wideband waveforms • Determine specific frequencies within the band at which interference is present • Identify a clear sub-band (if present) of the allocated band, within which data transmission can be more effectively achieved • Support prediction of SNR achievable within a portion of  the band, for selection of waveform parameters as part of adaptive selection of modulation and coding • Be suitable for incorporation into a ‘4G’ ALE protocol – Timing characteristics suitable for use while scanning

Sp e c t r u m s e n s in g p r o t o t y p e   • Part of the prototype radio system Harris has used for WBHF waveform on-air testing • Prototype radio is commanded to compute energy spectrum of received RF signal • Spectrum dumped to PC application for display • Can compute spectrum while receiving WBHF transmission

Sp e c t r u m s e n s i n g f ie l d t e s t   ob serv at ions –c ha nn el nois e floor 

• Channel noise floor without interference

Fi e ld t e s t o b s e r v a t i o n s –   c h a n n e l n o is e w / 3 k H z p r o b e  

• Visual comparison provides a rough estimate of SNR; adjust for bandwidth to predict SNR with transmissions of different bandwidths – or estimate based on Received Signal Strength measurements

Fi e ld t e s t o b s e r v a t i o n s –   c hannel interference 

• Prominent AM broadcast interferer with other narrowband interference

Contribut ion to us e of HF  w id eb an d w a ve fo rm s   6 kHz

• Avoiding the AM broadcast interferer allowed us to transmit at 12800 bps error-free

A n o t h e r in t e r fe r e r …  

• Spectrum sense of assigned frequency 6060 • AM broadcast signal is CFRX 1kW Toronto talk radio AM broadcast • Modem experienced 50% BER for 24 kHz 64000 bps

… avoided! 

12 kHz

• We retuned carrier frequency to 6054, reduced bandwidth to 12kHz • Achieve error-free data transfer at 32000bps • Spectrum above shows our signal and the AM broadcast

Caveats 

• Need to combine probing with spectrum sensing to be able to predict SNR • Spectrum sensing for WB-ALE will be subject to the well-known hidden node problem (and ‘exposed node problem’) affecting media access control protocols – May motivate incorporating some sort of cooperative technique … – … which might significantly impact the complexity of  the ALE protocols … – … so it would be great to know how important these problems are for us!

Con c lus ion s a nd fut ur e w or k   •  The introduction of the new HF wideband waveforms will give rise to needs for new ALE capabilities – Establish links of different bandwidth in a coordinated manner – For a given allocated channel, determine in real time what portion of the allocated band is usable – Support adaptive selection of waveform parameters •  The new ALE system will need a spectrum sensing capability having particular features – Identify interference-free portion of an allocated wideband channel – Provide estimate of achievable SNR

• Harris has developed a prototype of a spectrum sensing capability and demonstrated its utility for WBHF traffic on real-world HF skywave links • In future work, we hope to – Further elaborate the use of spectrum sensing as a component of a wideband ALE solution – Further investigate the significance of the spectrum sensing hidden-node problem through additional field testing and analysis